Some such power supply apparatuses include an input-side AC-to-DC converter which converts a commercial AC voltage into a DC voltage. The DC voltage is then converted into a high-frequency voltage in an inverter, and the high-frequency voltage is voltage-transformed by a transformer. The voltage-transformed voltage is then converted back into a DC voltage in an output-side high-frequency-to-DC converter. The resulting DC voltage is applied to an arc-utilizing apparatus. The transformer can be small in size because voltage-transforming is carried out after a DC voltage is converted into a high-frequency voltage. This, in turn, enables down-sizing of the power supply apparatus.
When, for example, an input voltage is of the order of four hundred volts (400 V), a voltage as high as at least 400 V.times.2.congruent.565V may be applied to the inverter. Then, IGBTs or MOSFETs used in the inverter as its switching devices must have withstand voltage of 1200 V or higher. Fewer switching devices having a withstand voltage of 1200 V or higher are commercially available relative to switching devices having a withstand voltage of 600 V or so. In addition, one switching device having a withstanding voltage of the order of 1200 V is more expensive than two switching devices having a withstanding voltage of 600 V. The switching frequency at which a switching device having a withstand voltage of the order of 600 V can be switched can be higher than the switching frequency for a 1200 V withstand voltage switching device. Accordingly, a transformer succeeding such inverter formed of 600 V withstand voltage switching devices can be smaller, which, in turn, makes it possible to manufacture a smaller sized power supply apparatus.
In U.S. Pat. No. 5,272,313 issued on Dec. 21, 1993 and assigned to the same assignee as the present application, a power supply apparatus which is small in size and can be manufactured at a low cost has been proposed. The power supply apparatus disclosed in this U.S. patent can receive a high input voltage by virtue of using a series combination of two inverters formed by switching devices having a withstand voltage of the order of 600 V.
The power supply apparatus of the U.S. patent is schematically shown in FIG. 1. A three-phase commercial AC voltage of the order of, for example, 400 V is applied to input power supply terminals 2a, 2b and 2c. The input AC voltage is, then, rectified by an input-side rectifier 4 in the form of, for example, a diode-bridge configuration. Instead of the three-phase AC voltage, a single-phase AC voltage may be applied to the input power supply terminals.
The input-side rectifier 4 has two output terminals, namely, a positive output terminal P and a negative output terminal N, between which a pair of smoothing capacitors 6a and 6b are connected in series to smooth the rectifier output voltage into a DC voltage.
An inverter 8a is connected across the capacitor 6a, and an inverter 8b is connected across the capacitor 6b. The inverters 6a and 6b form a DC-to-high-frequency converter. The inverters 8a and 8b include semiconductor switching devices, e.g. IGBTs 10a and 12a, and IGBTs 10b and 12b, respectively. The IGBTs 10a, 10b, 12a and 12b have a withstand voltage of the order of, for example, 600 V. The collector-emitter paths of the IGBTs 10a and 12a of the inverter 8a are connected in series, and a series combination of capacitors 14a and 16a is connected in parallel with the series combination of the IGBTs 10a and 12a. Flywheel diodes 18a and 20a are connected in parallel with the collector-emitter paths of the IGBTs 10a and 12a, respectively, with their anodes connected to the emitters of the respective IGBTs and with their cathodes connected to the collectors.
The inverter 8b also includes capacitors 14b and 16b and flywheel diodes 18b and 20b, which are connected in the same manner as the capacitors 14a and 16a and the flywheel diodes 18a and 20a of the inverter 8a. The inverters 8a and 8b convert a DC voltage inputted thereto to a high-frequency voltage.
A primary winding 22aP of a high-frequency transformer 22a has its two ends connected to the junction of the IGBTs 10a and 12a, which provides an output terminal of the inverter 8a, and the junction of the capacitors 14a and 16a. Also, a primary winding 22bP of a high-frequency transformer 22b has its two ends connected to the junction of the IGBTs 10b and 12b, which provides an output terminal of the inverter 8b, and the junction of the capacitors 14b and 16b. The transformers 22a and 22b form the rest of the DC-to-high-frequency converter.
The anodes of output-side rectifying diodes 24a and 26a are connected to opposite ends of a secondary winding 22aS1 of the transformer 22a, and the anodes of output-side rectifying diodes 24b and 26b are connected to opposed ends of a secondary winding 22bS1 of the transformer 22b. The cathodes of the four rectifying diodes 24a, 24b, 26a and 26b are connected together to a positive load output terminal 30P through a smoothing reactor 28. Intermediate taps on the secondary windings 22aS1 and 22bS1 are connected together to a negative load output terminal 30N. An arc-utilizing apparatus is connected between the output terminals 30P and 30N. With this arrangement, high-frequency voltages induced across the secondary windings 22aS1 and 22bS1 are converted to a DC voltage, which, in turn, is applied to the arc-utilizing apparatus.
A load current detector 32 is connected between the junction of the intermediate taps of the secondary windings and the negative load output terminal 30N, to detect a load current and produce a load-current representative signal representing the load current. The load-current representative signal is applied to an error amplifier 34, to which also applied is a load-current setting signal from an output-current setting device 36. The output-current setting device 36 is used to set the level of the output current supplied to the load. The error amplifier 34 develops an error signal representing the difference between the load-current representative signal and the load-current setting signal, which is applied to inverter control units 38a and 38b. The inverter control unit 38a provides a control signal to the IGBTs 10a and 12a for controlling the conduction period of the IGBTs 10a and 12a, while the inverter control unit 38b provides a control signal to the IGBTs 10b and 12b for controlling the conduction period of the IGBTs 10b and 12b. These connections provide a feedback control to automatically make the load current equal to the load current as represented by the load-current setting signal.
The transformers 22a and 22b have another secondary windings 22aS2 and 22bS2, respectively. A diode bridge formed by diodes 40a, 42a, 44a and 46a has its input terminals connected to the two ends of the secondary winding 22aS2, has its one output terminal connected through a resistor 48a to one end of the smoothing capacitor 6b and has its other output terminal connected to the other end of the capacitor 6b. Similarly, a diode bridge formed by diodes 40b, 42b, 44b and 46b has its input terminals connected to the two ends of the secondary winding 22bS2, has its one output terminal connected through a resistor 48b to one end of the smoothing capacitor 6a and has its other output terminal connected to the other end of the capacitor 6a.
Input voltages to the inverters 8a and 8b would sometimes differ due to difference in capacitance and leak current of the capacitors 6a and 6b. However, with the above-described arrangement, the input voltages can be balanced. For example, when the input voltage to the inverter 8a is higher than the input voltage to the inverter 8b, the voltage applied across the primary winding 22aP of the transformer 22a is higher than the voltage across the primary winding 22bP of the transformer 22b, resulting in a higher voltage induced across the secondary winding 22aS2 than across the secondary winding 22bS2. The higher induced voltage is applied across the smoothing capacitor 6b which has provided the lower output voltage, while the lower induced voltage is applied across the smoothing capacitor 6a which has provided the higher output voltage. As a result, the input voltages to the inverters 8a and 8b are balanced with respect to each other. The same can be said when the input voltage applied to the inverter 8a is lower than the input voltage to the inverter 8b.
If it is desired to correct imbalance of the input voltages to the inverters 8a and 8b at a high rate, low resistance resistors must be used as the resistors 48a and 48b. Also, the resistors 48a and 48b have to conduct a large current like the one flowing in the inputs of the inverters 8a and 8b. Such low resistance, large current conducting resistors should be large in size, which cancels out the downsizing realized by the use of the inverters 8a and 8b.
Therefore, an object of the present invention is to provide a power supply apparatus which can rapidly correct imbalance in voltage and can still be small in size.